"One day he will be the big lumbering 3 year old that hits. Best not be too smug."
"The parents of younger children really don't like the older kids being there doing older kid things. When your son is about 3 years old you will be the other parent…"
I don't agree with this. DS was never that child, even if other children hurt him first. He's never hit another child, and I wouldn't call wilfully hitting someone an older kid thing. Running about being boisterous yes, that's an older child thing, but deliberate violence, no.
The OP might never be that other parent because not every child attacks others, certainly not on a regular basis.
When DS was three, he was given a black eye and bitten and scratched by a girl a bit older than him, while we were in a soft play area of a cafe at a children's play park. He had teeth marks under his eye and scratches all over his face and his hand was bitten.
Adults weren't allowed into the area, but we were watching from the side and went in anyway. He was too far away for us to reach him before she bit and scratched him, as it all happened very quickly and we had to run around a soft mesh fence to the entry bit, and DS and the girl were at the back.
However her grandparents were at the other side of the cafe and not even looking, they were sitting with their backs to her and behind a raised partition.
After we pulled DS away, she attacked another child about the same age, and a ten month old baby whose mother was sitting with him in the separate area for babies. The baby had scratches down both sides of his face where she raked her nails down it. The other child was bitten on the hand and cheek.
It took three groups of parents complaining to staff to get her grandmother into the play area to remove her, and then they comforted her for being upset at being made to come out.
DH went over to speak to them and took DS to show them the state of his face and all they had to say was "well she always does it, we've told her not to, but she always does. What do you want us to do, keep her at home?"
We'd have liked them to be keeping a proper eye on her, not sitting at the other side of the room with their backs to her.
Failing that, we'd have liked them to apologise and actually tell her off properly. Instead they said "Did you bite this little boy? How would you like it if we bit you?…Oh…don't cry, it's alright…" and comforted her again.
If that child wasn't a bully already, she probably is by now, as nobody seemed to care about her behaviour. He grandparents seemed worn out by her, but were happy to let other children be hurt while they took a break and let her run wild. If that has carried on, she'll never have a reason to change because she has no consequences or boundaries.
There were probably over 100 children in that play area, and only one of them was attacking the others. So I don't think it's fair to say that one day the OP's child will be doing the same, as not all children are like this even at a young age, and it's not part of all older children's behaviour either.